69 Mustang kills after 45 mph - MustangForums.com



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Old 07-28-2017, 08:18 PM   #1
showoffzx2
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Default 69 Mustang kills after 45 mph

Hey all,

1969 Mustang 5.0 with a stock 2 barrel carb. Carb was replaced about 8 years ago. The last two years the car was barely driven. Seafoam was in the tank the whole time. PM was done on the car around the same time.

Fast forward till now. Starts up, idles percent, revs fine. Anything below 45 mph its got power. Anything over it, it falls flat on it face and either almost dies or does die.I can start back up no problem and drive it again.

I replaced fuel filter thinking it was plugged. That didnt change anything. I doubt fuel pump since I would think it would have a hard time starting. Leads me to believe the carb or air is getting cut off. Its using the factory air cleaner with the exhaust manifold heat opening a spring to the air cleaner. Its factory and I would think it would choke at starting or right away in the rpm range.

Thoughts?
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Old 07-28-2017, 08:45 PM   #2
Gun Jam
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Im totally down for fuel pump.

Here is why:

As speed increases so does load on the engine this translates to an increase in fuel consumption (although 45mph is still quite small its more than at idle) Its very possible that the fuel pump is so bad that its not capable of keeping up with the even modest demand at 45 mph.

Some ways to check:

1) with the engine warmed up begin at a stop then apply full power with in 1 second and hold it at WOT. I suspect that it will pull well for 2 or 3 seconds then fall on its face.

2) Once the engine dies or runs very rough shift into N and shut off the motor, coast to a stop (obviously plan for this and do it some place safe) remove the air cleaner (can be done prior to test) and look down the carb and by hand move the throttle quickly forward to wide open. Do you see any fuel streams from the accelerator pump. If not this would indicate that the carb is out of fuel which would confirm the fuel delivery issue. You should do a before and after check start the car in the drive way let it idle for 30 sec then shut it off then remove the air cleaner and by hand run the throttle open quickly to full as you watch down the carb. You will see a stream of fuel shoot into the carb barrel when you do this (or you should) does it look the same after the issue occurs you shift into N and kill the engine as I described? killing the engine and Shifting into N is important because it stop the wheels from rolling the engine over and running the fuel pump causing the floats to refill. (we are really just checking for dry floats using the accel pump to check for fuel) You should get 2 good shots out of it. so move to WOT twice.

3) sitting for long periods is murder on the fuel pump diaphragms

4) If it is an air issue the only culprit would be a horrible rat nest air cleaner OR a choke plate that is falling closed and reopening. remove the air cleaner and verify the choke plate is open all the way and will not move shut..If its an electric choke check with hot engine.

Last edited by Gun Jam; 07-28-2017 at 08:48 PM.
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Old 07-28-2017, 09:37 PM   #3
showoffzx2
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Quote:
Originally Posted by Gun Jam View Post
Im totally down for fuel pump.

Here is why:

As speed increases so does load on the engine this translates to an increase in fuel consumption (although 45mph is still quite small its more than at idle) Its very possible that the fuel pump is so bad that its not capable of keeping up with the even modest demand at 45 mph.

Some ways to check:

1) with the engine warmed up begin at a stop then apply full power with in 1 second and hold it at WOT. I suspect that it will pull well for 2 or 3 seconds then fall on its face.

2) Once the engine dies or runs very rough shift into N and shut off the motor, coast to a stop (obviously plan for this and do it some place safe) remove the air cleaner (can be done prior to test) and look down the carb and by hand move the throttle quickly forward to wide open. Do you see any fuel streams from the accelerator pump. If not this would indicate that the carb is out of fuel which would confirm the fuel delivery issue. You should do a before and after check start the car in the drive way let it idle for 30 sec then shut it off then remove the air cleaner and by hand run the throttle open quickly to full as you watch down the carb. You will see a stream of fuel shoot into the carb barrel when you do this (or you should) does it look the same after the issue occurs you shift into N and kill the engine as I described? killing the engine and Shifting into N is important because it stop the wheels from rolling the engine over and running the fuel pump causing the floats to refill. (we are really just checking for dry floats using the accel pump to check for fuel) You should get 2 good shots out of it. so move to WOT twice.

3) sitting for long periods is murder on the fuel pump diaphragms

4) If it is an air issue the only culprit would be a horrible rat nest air cleaner OR a choke plate that is falling closed and reopening. remove the air cleaner and verify the choke plate is open all the way and will not move shut..If its an electric choke check with hot engine.
Thank you for the post. I really appreciate it.

The car has no problem in WOT in 1st, 2nd gear once I hit 3rd gear that's when it falls flat on its face. In fact when it starts to fall on its face, I can hold the throttle and the motor surges like its trying to stay alive but its dying like and on and off switch being turned on and off.

What about a coil with spark with load at a higher gear. Could that be it?
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Old 07-28-2017, 09:48 PM   #4
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yeahhh..if its pulling hard to 3rd that does make the fuel pump less likley (if you get frustrated you can always come back and check)

did you check that the choke plate is secure?

sure it could be spark but the coil and distributor are mostly independent of load (minus the vac advance if you run one) The Coil and dizzy are highly effected by RPM...but you said it pulls great through 1st and 2nd when RPM is highest and runs worst when RPM is lower and load is higher which makes the ignition system even LESS likely than fuel.

Stopped in neutral when you smoothly rev the engine up to like 4,000 RPM (pretty dang fast) does it still run smooth or does it get rough? IF it runs very smooth at high rpm no load not likley ignition.

if the above paragraph is true (runs smooth) My bet is still on fuel
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Old 07-29-2017, 07:42 AM   #5
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yeahhh..if its pulling hard to 3rd that does make the fuel pump less likley (if you get frustrated you can always come back and check)

did you check that the choke plate is secure?

sure it could be spark but the coil and distributor are mostly independent of load (minus the vac advance if you run one) The Coil and dizzy are highly effected by RPM...but you said it pulls great through 1st and 2nd when RPM is highest and runs worst when RPM is lower and load is higher which makes the ignition system even LESS likely than fuel.

Stopped in neutral when you smoothly rev the engine up to like 4,000 RPM (pretty dang fast) does it still run smooth or does it get rough? IF it runs very smooth at high rpm no load not likley ignition.

if the above paragraph is true (runs smooth) My bet is still on fuel
If I am in the driveway parked, I can rev it all day and it's fine. I was thinking less likely on ignition as well since you would think it would act up in all gears not just a higher.

Thinking of Fuel, there are only three things that can effect it. Fuel Pump, Fuel Filter and Carb. Fuel filter is new and carb is newer. Fuel pump might be old, but you think it would act up in 1st and 2nd gear and have a hard time starting. I guess the old gas could effect it, but you think that would happen all the time.
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Old 07-29-2017, 10:42 AM   #6
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you can also pull the fuel line off and place it in a jar or container and crank the engine over and watch how the fuel surges into the container. You should get 1/2 cup of fuel in about 2 or 3 seconds.

It could be debris floating around in the float bowl thats getting caught in the jets
It could be obstructed inlets or issues with the inlet needles or floats.
We can speculate for days about this stuff but we'll get no where unless you can verify certain conditions.

I still suspect the float bowls are running dry or have debris in them.
You could pull the top off that carb and check everything.
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Old 07-30-2017, 03:07 PM   #7
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Quote:
Originally Posted by Gun Jam View Post
you can also pull the fuel line off and place it in a jar or container and crank the engine over and watch how the fuel surges into the container. You should get 1/2 cup of fuel in about 2 or 3 seconds.

It could be debris floating around in the float bowl thats getting caught in the jets
It could be obstructed inlets or issues with the inlet needles or floats.
We can speculate for days about this stuff but we'll get no where unless you can verify certain conditions.

I still suspect the float bowls are running dry or have debris in them.
You could pull the top off that carb and check everything.
When I get more time, I will have to investigate it. I took it around several blocks keeping it under 40mph to see if anything would happen. It ran perfectly fine.
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Old 07-30-2017, 05:26 PM   #8
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yep it still sounds fuel related...normally an ignition misfire bad enough to kill an engine will show up at any speed.
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Old 08-09-2017, 03:10 PM   #9
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Does this car have an automatic transmission?
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Old 08-11-2017, 09:03 PM   #10
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Following up on this:

After doing a compression test on the motor (came out great!), I noticed a good size leak coming from the accelerator pump. I am going to get a new one and my guess is that it should be good to go.

Now I am on the other problem. I did a compression test today and now it's acting like a misfiring. Now I need to go back in the garage and figure that one out.
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